Instrument

EVE Overview

The Extreme ultraviolet Variabilty Experiment (EVE) is designed to measure the solar extreme ultraviolet (EUV) irradiance. The EUV radiation includes the 0.1-105 nm range, which provides the majority of the energy for heating Earth’s thermosphere and creating Earth’s ionosphere (ionized plasma). This wide spectral range requies the use of multiple channels. Some key requirements for EVE are to measure the solar EUV irradiance spectrum with 0.1 nm spectral resolution and with 20 sec cadence. These drive the EVE design to include grating spectrographs with array detectors so that all EUV wavelengths can be measured simultaneously. Another key requirement for EVE is to measure the EUV radiation with an accuracy of 25% or better, thus on-board calibration channels are included to go with underflight calibration experiments to be conducted during the SDO mission.

This is the solar EUV spectrum that includes many emission lines from the solar chromosphere, transition region, and corona

MEGS-SAM: pinhole camera used with MEGS-A CCD to measure individual X-ray photons in the 0.1 nm to 7 nm range

MEGS-P: photodiode used with the first grating in MEGS-B to measure the bright H I Lyman-alpha emission

ESP – Euv SpectroPhotometer: transmission grating spectrograph that measures 4 bands in the 17-38 nm range and also 0.1-7 nm in zeroth order. ESP provides calibrations for MEGS sensitivity changes and higher time cadence (0.25-second). The ESP is very similar to the SOHO SEM instrument.

EEB – EVE Electronics Box: electronics that control the MEGS and ESP instruments and provides an interface to/from the SDO spacecraft.

The EVE instrument is a fairly large instrument with its size being 100 cm (39″) long by 61 cm (24″) wide by 36 cm (14″) high and weighing 61 kg (135 lbs). Its average power is 60 Watts, but can peak up to 137 Watts. The engineering data are output at a low rate of 2 kilobits per sec (kbps), but the science data rate is at 7 megabits per sec (Mbps) in order to download both MEGS CCD images every 10 seconds.